1. Field of the Invention
This invention relates to a rotary magnetic head device employed in a magnetic recording/reproducing apparatus, such as a video tape recorder. More particularly, this invention relates to an arrangement of an electro-mechanical transducer element supported on a head base of a rotary drum, each formed of materials having substantially the same coefficient of linear expansion. Still more particularly, this invention relates to such an arrangement which suppresses the protrusion of the head base and the transducer element to provide a stable abutment between them.
2. Description of the Related Art
When a magnetic head device is loaded on, for example, a video tape recorder, a mechanism for displacing the magnetic head is required because it is necessary to provide a tracking means to cause the magnetic head to follow a recording track for tracking during variable speed reproduction such as fast feed or slow reproduction tracking. As such a tracking means for causing the magnetic head to follow the recording track, an electro-mechanical transducing element, such as a bimorph, is employed. The transducing element is comprised of two polarized piezoelectric plates cemented together with a reinforcement plate of titanium, for example, interposed between the two piezoelectric plates.
As an example of the above-mentioned magnetic head device employing the above-mentioned electromechanical transducer element, a device disclosed in, for example, JP Patent Kokai (Laid-Open) Patent Publication No. 55-19393 (1980) has been proposed. The magnetic head device has its proximal portion secured to a head base made of an aluminum material for supporting an electro-mechanical transducer element near an end in a cantilevered fashion which has its distal or free end fitted with a magnetic head. The electro-mechanical transducer element is displaced when a driving voltage is applied to the transducer element for changing the position of the magnetic head carried at its distal end.
The magnetic head device has a control driving circuit for detecting the amount of displacement of the magnetic head and for changing the position of the magnetic head depending on the amount of such displacement. The control driving circuit includes a displacement amount detection circuit for detecting a change in resistance responsive to the amount of displacement of the transducer element from a strain gauge provided between the fixed proximal and free distal ends of the transducer element for functioning as a sensor. The control driving circuit also includes a bimorph driving circuit for applying a driving voltage to the transducer element.
An electromechanical transducer element, such as the bimorph element of two polarized piezoelectric plates cemented together is arranged on the head base, with a reinforcement plate of titanium interposed between the two piezoelectric plates. The rotary drum and the base head are formed of an aluminum material, and the electromechanical transducer element is formed of, for example, a piezoelectric ceramic material, as discussed above. The aluminum material and the piezoelectric ceramic material have linear expansion coefficients .alpha..sub.A, .alpha..sub.c of 23.6.times.10.sup.-6 and 6.times.10.sup.-6 (l/C), respectively.
If the distance between the proximal fixed end and the distal free end of the cantilevered electromechanical transducer element is s.sub.1, the length between the base head secured fixed end and the distal free end of the magnetic head is s.sub.2 and the distance between the outer periphery of the rotary drum and the distal end of the magnetic head is H, a variation .DELTA.H of the distance H caused with a temperature change AT is represented by an equation EQU .DELTA.H=(s.sub.1 .times..alpha..sub.c s.sub.2 .times..alpha..sub.A).times..DELTA.T (1)
as a function of the above-mentioned linear expansion coefficients. The variation .DELTA.H is changed depending upon the temperature, as indicated by the formula (1). In effect, the variation .DELTA.H represents a protruding amount of the magnetic head relative to the outer periphery of the rotary drum. It is apparent from the equation (1) that the variation .DELTA.H indicating the amount of protrusion of the magnetic head is produced by the difference in the linear expansion coefficients of the respective materials of the component parts under the same temperature conditions dand environment.
If the amount of protrusion of the magnetic head is plotted against the drum temperature, the amount of protrusion of the magnetic head is .DELTA.H.sub.0 and .DELTA.H.sub.HI for a drum temperature of 0.degree. C. and higher than 60.degree. C., respectively. If aluminum material and piezoelectric ceramic material are respectively employed, the amounts of protrusion .DELTA.H.sub.0 and .DELTA.H.sub.HI are 50 to 60 .mu.m and 10 to 20 .mu.m, respectively. Consequently, a difference of approximately 40 .mu.m is produced in the amount of protrusion of the magnetic head with a difference in temperature, wherein the amount of protrusion tends to be decreased with higher temperatures.
Such a difference in the amount of protrusion causes a differential abutment of the magnetic head with respect to the magnetic tape as a recording medium. That is, the magnetic head is receded with respect to the rotary head at higher temperatures so that the magnetic head is poorly abutted against the magnetic tape. Conversely, the magnetic head is protruded with respect to the rotary head at lower temperatures, thus causing damage to the magnetic tape under the effects of vibrations.
If the rotary drum is reduced in size, as in an extreme case as shown in FIG. 5, a length of the electromechanical transducer element is required from the fixed end for maintaining a sufficient amount of elastic deformation. If the requirement in length is met, and the distal end of the magnetic head 23 is arranged with a pre-set amount of protrusion on the rotary drum 28 as shown for example in FIG. 5, the magnetic head device is fixed on the rotary axis of the motor arranged at the center position of the rotary drum 28. With such a fixed position, the magnetic head device cannot be fixed on the disc of the rotary drum 28, with the result that the rotary drum 28 cannot be reduced in size.
Accordingly, it is a continuing problem in the art to provide a rotary magnetic head device with a proximal portion secured to a head base of a rotary drum for supporting an electromechanical transducer element in a manner in which protrusions of the magnetic head device with changes of temperature of the rotary drum continue to permit the head device to contact recording medium in a stable tracking manner.